The present invention generally relates to orthopedic implants used for correction of spinal injuries or deformities, and more specifically, but not exclusively, concerns apparatuses for fixing a portion of the spine, such as the cervical spine, to allow correction or healing thereof.
In the realm of orthopedic surgery, it is well known to use implants to fix the position of bones. In this way, the healing of a broken bone can be promoted, and malformations or other injuries can be corrected. For example, in the field of spinal surgery, it is well known to place such implants into vertebrae for a number of reasons, including (a) correcting an abnormal curvature of the spine, including a scoliotic curvature, (b) to maintain appropriate spacing and provide support to broken or otherwise injured vertebrae, and (c) perform other therapies on the spinal column.
Typical implant systems include several pieces, which commonly are useful and may be associated with only specific other pieces. Bone screws, hooks, and clamps are well know as fixation devices, which are connected or adjoined to a particular bone as a connection between the remainder of the implant and the bone. Specially formed plates or rods are commonly used as stabilization and support members. Thus, in a common spinal implant system, a spinal plate is implanted along one or more vertebrae by driving a bone screw through the plate and into each of two vertebrae. The vertebrae are thus supported and kept in a particular position by the plate, so as to promote healing. One example of such an instrumentation system is U.S. Pat. No. 5,735,853 to Olerud.
Alternatively, a rod can be used as the support and stabilizing member. In such an implant, a series of two or more screws are inserted into two or more vertebrae to be instrumented. A rod is then placed within or coupled to the heads of the screws, or is placed within a connecting device that links the rod and a screw head, and the connections are tightened. In this way, a rigid supporting structure is fixed to the vertebrae, with the rod providing the support that promotes correction of the vertebral malformation or injury.
Many varieties of bone fixation devices (e.g. screws and hooks) are monoaxial in construction. That is, such devices are connected to the rod or plate such that a longitudinal axis through the rod or plate and a longitudinal axis through the fixation device are capable of only a single position with respect to each other. While useful in certain circumstances, in many therapeutic situations the degree of precision required to use such an inflexible device is impractical, or can lead to a longer duration of surgery, potentially awkward angles for the surgeon and for the patient, with the potential for attendant complications such as pain and/or extended rehabilitation.
More recently, bone fixation devices having multi-axial capability have been introduced. Examples of such constructs are shown in U.S. Pat. Nos. 5,797,911, 5,954,725, and 5,810,818. These devices help to reduce the required precision of placement of the fixation device, since a head portion of the fixation device is multi-axially positionable around the bone-threaded or hook portion. The head can thus be positioned so as to easily receive the rod, limiting or removing much of the positioning difficulty inherent in prior devices.
Most such devices are designed for spinal fixation at the thoracic and lumbar levels. Accordingly, there is a need in the art for a comprehensive multi-axial spinal implant system, and particularly one that is useful in the cervical region of the spine.